A variety of materials have heretofore been proposed and, particularly, polyvinylidene chloride, polyacrylonitrile and ethylene/vinyl alcohol copolymer have been known as gas barrier resins. From the standpoint of environmental problems, however, it is a trend to refrain from using the polyvinylidene chloride and polyacrylonitrile. The ethylene/vinyl alcohol copolymer, on the other hand, has gas barrier property that greatly varies depending on the humidity, i.e., has a problem of a decrease in the gas barrier property under highly humid conditions.
In order to impart gas barrier property to the packing materials, there has been known a method of using films obtained by depositing an inorganic material on the surface of a base material. Such films, however, are very costly and involve problems in regard to flexibility of the vapor-deposited film and adhesiveness to the base material.
In order to solve the above problems, there have been proposed a gas barrier resin composition obtained by reacting a polycarboxylic acid polymer, a crosslinking agent having 2 to 4 functional groups that react with the carboxyl groups, and metal ions of a valency of two or more so that the polycarboxylic acid polymer forms crosslinked portions due to the crosslinking agent and crosslinked portions due to the metal ions having a valency of two or more, the weight ratio of the polycarboxylic acid polymer and the crosslinking agent being from 99.9/0.1 to 65/35 (patent document 1); and a gas barrier film obtained by forming a gas barrier coating on at least one surface of a thermoplastic resin film, the coating being formed of a polyacrylic acid that is crosslinked with a crosslinking agent containing an epoxy compound having three or more epoxy groups per a molecule thereof, and the crosslinking agent being contained in an amount of 1 to 100 parts by mass per 100 parts by mass of the polyacrylic acid (patent document 2).
The gas barrier materials disclosed in the above patent documents 1 and 2 must be crosslinked to a high degree at a high temperature of not lower than 150° C. or by being heated for extended periods of time accompanied by such problems as seriously affecting the plastic base material, and requiring a dip treatment or a spray treatment at the time of ionic crosslinking with metal ions, lowering the productivity, and consuming large amounts of energy and water. Besides, the gas barrier materials are not still satisfactory in regard to flexibility and retort resistance.
As a gas barrier laminated film on which dry-printing can be effected at a relatively low temperature, further, there has been proposed a gas barrier laminated film comprising a layer (a) formed from a coating solution (A) containing a polycarboxylic acid polymer without heat treatment and a layer (b) formed from a coating solution (B) containing a water-soluble polyvalent metal salt and an aqueous resin, the layer (a) of the coating solution (A) and the layer (b) of the coating solution (B) forming at least a pair of lamination units abutting to each other (patent document 3).
The gas barrier laminated film disclosed in the above patent document 3 can be dry-printed at a relatively low temperature; i.e., the gas barrier laminated film is obtained without affecting the base material. With this laminated film, however, the polycarboxylic acid polymer that is applied first is fixed and does not sufficiently migrate into the layer (a) of the polyvalent metal layer. Therefore, the crosslinking with polyvalent metal ions is not attained to a sufficient degree, and the gas barrier property is not still satisfactory unless the ionic crosslinking ratio is enhanced by employing a dip treatment or a spray treatment which is the conventional method as the treatment after the printing.
The present inventors have, further, proposed a gas barrier material having a barrier layer comprising a polycarboxylic acid polymer, the surface layer of the barrier layer forming a chemical bond stemming from an isocyanate group, and the content of nitrogen in the surface layer being at least 1 atom % or more per the total amount of carbon, oxygen and nitrogen (patent document 4).
The above gas barrier material is obtained by applying a gas barrier layer-forming coating material containing a high hydrogen-bonding polymer onto an isocyanate compound-containing layer that has been formed in advance and, thereafter, vaporizing the solvent in the coating material so that a surface layer forming a chemical bond stemming from the isocyanate group is formed on the barrier layer. Therefore, the gas barrier material offers an excellent effect of forming an anti-blocking layer with the isocyanate compound-containing layer as an anchoring layer in addition to exhibiting excellent gas barrier property, retort resistance and producibility.